Virus Structure and Method of Invasion

• http://videos.howstuffworks.com/tlc/29852-understanding-viruses-video.htm

• Viruses called bacteriophages– Can infect and set in motion a genetic

takeover of bacteria, such as Escherichia coli

Figure 18.10.5 m

• Bacteriophages, also called phages– Have the most complex capsids

found among viruses

Figure 18.4d

80 225 nm

50 nm(d) Bacteriophage T4

DNA

Head

Tail fiber

Tail sheath

• Recall that bacteria are prokaryotes– With cells much smaller and more

simply organized than those of eukaryotes

• Viruses Are smaller and simpler still

Figure 18.20.25 m

Virus

Animalcell

Bacterium

Animal cell nucleus

• Obligate parasites --A virus has a genome but can reproduce only within a host cell

•Reproduction is their only true characteristic of being alive

• Specific to type of cells they target-poliomylelitis virus attacks nerve cells-hepatitis virus attacks liver cells

Viral Structure

• Not a cell!• Hijacks biochemical machinery of

host cell to carry out processes necessary to reproduce

• Obligate intracellular parasite

Viral Genomes

• Viral genomes may consist of– Double- or single-stranded DNA– Double- or single-stranded RNA

• Classes of animal viruses

Table 18.1

Figure 18.4a, b

18 250 mm 70–90 nm (diameter)

20 nm 50 nm(a) Tobacco mosaic virus (b) Adenoviruses

RNADNACapsomere

Glycoprotein

Capsomereof capsid

Capsids and Envelopes

• A capsid– Is the protein shell that encloses the viral genome

– Can have various structures

• Some viruses have envelopes– Which are membranous coverings

derived from the membrane of the host cell

Figure 18.4c

80–200 nm (diameter)

50 nm(c) Influenza viruses

RNA

Glycoprotein

Membranousenvelope

Capsid

Bacteriophage

Invasion of a cell by a virus

• Virus can lie dormant for many years until it comes into contact with a suitable host cell

• Binds with molecules on surface of host cell

• Herpes-whole virus enters cell

• Bacteriophage-viral DNA injected via hollow tail

Alteration of Cell Instruction

• Virus takes control of cell machinery• Depends on host for

-ATP-supply of free nucleotides

• Virus suppresses cell’s normal nucleic acid replication and protein synthesis

• Manufactures many identical copies of viral nucleic acid and protein coats

The Lytic Cycle

• The lytic cycle– Is a phage reproductive cycle that

culminates in the death of the host– Produces new phages and digests the

host’s cell wall, releasing the progeny viruses

• The lytic cycle of phage T4, a virulent phage

Phage assembly

Head Tails Tail fibersFigure 18.6

Attachment. The T4 phage usesits tail fibers to bind to specificreceptor sites on the outer surface of an E. coli cell.

1Entry of phage DNA and degradation of host DNA.The sheath of the tail contracts,injecting the phage DNA intothe cell and leaving an emptycapsid outside. The cell’sDNA is hydrolyzed.

2

Synthesis of viral genomes and proteins. The phage DNAdirects production of phageproteins and copies of the phagegenome by host enzymes, usingcomponents within the cell.

3Assembly. Three separate sets of proteinsself-assemble to form phage heads, tails,and tail fibers. The phage genome ispackaged inside the capsid as the head forms.

4

Release. The phage directs productionof an enzyme that damages the bacterialcell wall, allowing fluid to enter. The cellswells and finally bursts, releasing 100 to 200 phage particles.

5

The Lysogenic Cycle

• The lysogenic cycle– Replicates the phage genome without

destroying the host

• Temperate phages– Are capable of using both the lytic and

lysogenic cycles of reproduction

• The lytic and lysogenic cycles of phage , a temperate phage

Many cell divisions produce a large population of bacteria infected with the prophage.

The bacterium reproducesnormally, copying the prophageand transmitting it to daughter cells.

Phage DNA integrates into the bacterial chromosome,becoming a prophage.

New phage DNA and proteins are synthesized and assembled into phages.

Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle.

Certain factorsdetermine whether

The phage attaches to ahost cell and injects its DNA.

Phage DNAcircularizes

The cell lyses, releasing phages.Lytic cycleis induced

Lysogenic cycleis entered

Lysogenic cycleLytic cycle

or Prophage

Bacterialchromosome

Phage

PhageDNA

Figure 18.7

Retrovirus

• Contains RNA• No DNA to transcribe into mRNA

• Reverse transcriptase injected into cell by virus with RNA-enzyme reverses normal transcription-Produces viral DNA from RNA-Virus uses DNA to replicate

Some video clips

• Bacteriophage entering a cell

• HIV replication

• http://www.youtube.com/watch?v=HhhRQ4t95OI

Emerging Viruses

• Emerging viruses– Are those that appear suddenly or suddenly

come to the attention of medical scientists

• Severe acute respiratory syndrome (SARS)– Recently appeared in China

Figure 18.11 A, B

(a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS.

(b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from the envelope.

• Outbreaks of “new” viral diseases in humans– Are usually caused by existing viruses that expand

their host territory

Viral Diseases in Plants• More than 2,000 types of viral diseases of

plants are known• Common symptoms of viral infection include

– Spots on leaves and fruits, stunted growth, and damaged flowers or roots

Figure 18.12

• Plant viruses spread disease in two major modes– Horizontal transmission, entering through

damaged cell walls – Vertical transmission, inheriting the virus from

a parent

Viroids and Prions: The Simplest Infectious Agents

• Viroids– Are circular RNA molecules that infect plants

and disrupt their growth

• Prions– Are slow-acting, virtually indestructible

infectious proteins that cause brain diseases in mammals

– Propagate by converting normal proteins into the prion version

Figure 18.13

Prion

Normalprotein

Originalprion

Newprion

Many prions

• Concept 18.3: Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria

• Bacteria allow researchers– To investigate molecular genetics in the

simplest true organisms

The Bacterial Genome and Its Replication

• The bacterial chromosome– Is usually a circular DNA molecule with few

associated proteins

• In addition to the chromosome– Many bacteria have plasmids, smaller circular

DNA molecules that can replicate independently of the bacterial chromosome

Operons: The Basic Concept

• In bacteria, genes are often clustered into operons, composed of– An operator, an “on-off” switch– A promoter– Genes for metabolic enzymes

• Bacterial cells divide by binary fission– Which is preceded by replication of the bacterial

chromosomeReplicationfork

Origin of replication

Termination of replication

Figure 18.14

Mutation and Genetic Recombination as Sources of

Genetic Variation• Since bacteria can reproduce rapidly– New mutations can quickly increase a

population’s genetic diversity

• An operon– Is usually turned “on”– Can be switched off by a protein called a

repressor

• The trp operon: regulated synthesis of repressible enzymes

Figure 18.21a

(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes.

Genes of operon

Inactiverepressor

Protein

Operator

Polypeptides that make upenzymes for tryptophan synthesis

Promoter

Regulatorygene

RNA polymerase

Start codon Stop codon

Promoter

trp operon

5

3mRNA 5

trpDtrpE trpC trpB trpAtrpRDNA

mRNA

E D C B A

DNA

mRNA

Protein

Tryptophan(corepressor)

Active repressor

No RNA made

Tryptophan present, repressor active, operon off. As tryptophanaccumulates, it inhibits its own production by activating the repressor protein.

(b)

Figure 18.21b

Repressible and Inducible Operons: Two Types of Negative Gene Regulation

• In a repressible operon– Binding of a specific repressor protein to the

operator shuts off transcription

• In an inducible operon– Binding of an inducer to an innately inactive

repressor inactivates the repressor and turns on transcription

• The lac operon: regulated synthesis of inducible enzymes

Figure 18.22a

DNA

mRNA

ProteinActiverepressor

RNApolymerase

NoRNAmade

lacZlacl

Regulatorygene

Operator

Promoter

Lactose absent, repressor active, operon off. The lac repressor is innately active, and inthe absence of lactose it switches off the operon by binding to the operator.

(a)

5

3

mRNA 5'

DNA

mRNA

Protein

Allolactose(inducer)

Inactiverepressor

lacl lacz lacY lacA

RNApolymerase

Permease Transacetylase-Galactosidase

5

3

(b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.

mRNA 5

lac operon

Figure 18.22b

• Inducible enzymes– Usually function in catabolic pathways

• Repressible enzymes– Usually function in anabolic pathways

• Regulation of both the trp and lac operons– Involves the negative control of genes, because

the operons are switched off by the active form of the repressor protein

Positive Gene Regulation

• Some operons are also subject to positive control– Via a stimulatory activator protein, such as

catabolite activator protein (CAP)

Promoter

Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized.If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for the lactose pathway.

(a)

CAP-binding site OperatorRNApolymerasecan bindand transcribe

InactiveCAP

ActiveCAPcAMP

DNA

Inactive lacrepressor

lacl lacZ

Figure 18.23a

• In E. coli, when glucose, a preferred food source, is scarce– The lac operon is activated by the binding of a

regulatory protein, catabolite activator protein (CAP)

• When glucose levels in an E. coli cell increase– CAP detaches from the lac operon, turning it off

Figure 18.23b(b)Lactose present, glucose present (cAMP level low): little lac mRNA synthesized.

When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription.

Inactive lacrepressor

InactiveCAP

DNA

RNApolymerasecan’t bind

Operator

lacl lacZ

CAP-binding site

Promoter